In the production of vinyl halide polymers, it is common practice to stop the polymerization reaction by which these polymers are produced at a stage prior to that at which the reaction would ordinarily come to a halt in order to obtain stable uniform products having the desired porosity, melt viscosity, and other properties. Chain termination is usually effected by incorporating in the polymerization mixture a compound that will act as a chain-terminating or shortstopping agent for the polymerization reaction without adversely affecting the color of the vinyl halide polymer. The chain-terminating agent also serves to prevent polymerization of recovered monomer and to inhibit degradation of the polymer when it is heated during the monomer-removal steps and/or the drying step of the process.
In U.S. Pat. No. 2,662,867, Hoertz disclosed the use of water-soluble phenols, quinones, and nitro-aromatic compounds as chain-terminating agents in the preparation of vinyl chloride polymers. He taught that these compounds function as free radical scavengers that react with the free radicals that arise from residual catalyst, from active polymer chains, and from reaction between unreacted monomer and residual catalyst. He further indicated that the addition of these chain-terminating agents stabilizes the polymer not by reacting with the liberated hydrohalogen acid but rather by destroying the free radicals in the dispersion before polymer decomposition is inaugurated.
There are several disadvantages to the use of the chain-terminating agents disclosed by Hoertz that preclude their use in the commercial production of vinyl chloride polymers. Because they are soluble in water, these phenols are present as pollutants in the effluent water streams from the polymerization process, or if they are retained in the resin they are susceptible to water-extraction from the finished goods. In addition, because of their toxicity and corrosiveness, the water-soluble phenols present hazards to those who manufacture the polymeric products or handle the finished goods that contain these polymers.